#include <sys/wait.h>
#include <errno.h>
#include <signal.h>
#include <unistd.h>

/* Figure-8.22.c 一个简略版的system（未处理信号） */
/* system函数的 POSIX.1 正确实现 */
int
system(const char *cmdstring) /* with appropriate signal handling */
{
    pid_t            pid;
    int              status;
    struct sigaction ignore, saveintr, savequit;
    sigset_t         chldmask, savemask;

    if (cmdstring == NULL)
        return(1); /* always a command processor with UNIX */

    ignore.sa_handler = SIG_IGN; /* ignore SIGINT and SIGQUIT */
    sigemptyset(&ignore.sa_mask);
    ignore.sa_flags = 1;
    if (sigaction(SIGINT, &ignore, &saveintr) < 0)
        return(-1);
    if (sigaction(SIGQUIT, &ignore, &savequit) < 0)
        return(-1);
    sigemptyset(&chldmask); /* now block SIGCHLD */
    sigaddset(&chldmask, SIGCHLD);
    if (sigprocmask(SIG_BLOCK, &chldmask, &savemask) < 0)
        return(-1);

    if ((pid = fork()) < 0) {
        status = -1; /* probably out of process */
    } else if (pid == 0) { /* child */
        /* restore previous signal action & reset signal mask */
        sigaction(SIGINT, &saveintr, NULL);
        sigaction(SIGQUIT, &savequit, NULL);
        sigprocmask(SIG_SETMASK, &savemask, NULL);

        execl("/bin/sh", "sh", "-c", cmdstring, (char*)0);
        _exit(127); /* exec error */
    } else { /* parent */ 
        while(waitpid(pid, &status, 0) < 0)
            if (errno != EINTR) {
                status = -1; /* error other than EINTR from waitpid() */
                break;
            }
    }

    /* restore previous signal action & reset signal mask */
    if (sigaction(SIGINT, &saveintr, NULL) < 0)
        return(-1);
    if (sigaction(SIGQUIT, &savequit, NULL) < 0)
        return(-1);
    if (sigprocmask(SIG_SETMASK, &savemask, NULL) < 0)
        return(-1);

    return(status);
}
